The successful development of linear low density polyethylene (LLDPE) has forever changed the character of the polyethylene industry. For over fifty years, low density polyethylene (LDPE) was produced at pressures ranging up to 345 MPa (50,000 psi) and temperatures of about 300.degree. C. Technology was then developed in subsequent years which was capable of operating at less than 2 MPa (300 psi) and near about 100.degree. C. This technologic development has rapidly established itself as a low cost route to producing LLDPE.
LLDPE, which is typically made using a transition metal catalyst rather than a free-radical catalyst, as required for LDPE, is characterized by linear molecules having no long-chain branching; short-chain branching is instead present and is the primary determinant of resin density. The density of commercially available LLDPE typically ranges from 0.915-0.940 g/cm.sup.3. Moreover, commercially available LLDPEs generally exhibit a crystallinity of from about 25-60 vol. %, and a melt index which can range from 0.01 g/10 min. to several hundred g/10 min.
Many commercial LLDPEs are available which contain one or more comonomers such as propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and mixtures thereof. The specific selection of a comonomer for LLDPE is based primarily on process compatibility, cost and product design.
In today's polyethylene industry, LLDPEs are used in a wide variety of applications including film forming, injection molding, rotomolding, and wire and cable fabrication. A principal area for LLDPE copolymers is in film forming applications since such copolymers typically exhibit high dart impact, high Elmendorf tear, high tensile strength and high elongation, in both the machine direction (MD) and the transverse direction (TD), compared with counterpart LDPE resins.
Examples of previous developmental trends in this field include U.S. Pat. Nos. 5,260,245; 5,336,652; and 5,561,091, all to Mink et al., which disclose LLDPE films that exhibit the above properties made from polymerizing ethylene and at least one comonomer in the presence of a polymerization cocatalyst and vastly distinct transition metal catalysts. Specifically, in the '245 patent the transition metal catalyst is formed by treating silica having reactive OH groups with a dialkylmagnesium compound in a solvent; adding to said solvent a carbonyl-containing compound and then treating with a transition metal compound.
In the '652 patent, the transition metal catalyst is prepared by treating a support having a reduced surface OH content with an organomagnesium compound; treating the product with a silane compound having the formula R.sub.x.sup.1 SiR.sub.y.sup.2 wherein R.sup.1 is R.sub.w --O where R.sub.w is hydrocarbyl containing 1 to 10 carbon atoms; R.sup.2 is halogen, hydrocarbyl having 1 to 10 carbon atoms or hydrogen; x is 1, 2, 3 or 4 and y is 0, 1, 2 or 3 with the proviso that x+y=4, and a transition metal compound. In this reference, reduction of surface OH content of the silica is effectuated by heating or by treatment with an aluminum compound.
The transition metal catalyst employed in the '091 patent is one that is obtained by contacting silica having reactive OH groups with a dialkylmagnesium compound in a solvent; adding a mixture of an alcohol and SiCl.sub.4 thereto with subsequent treatment with a transition metal catalyst.
U.S. Pat. No. 4,335,016 to Dombro provides a supported olefin polymerization catalyst which is prepared by (1) forming a mixture of a calcined, finely divided porous support material and an alkyl magnesium compound; (2) heating the mixture for a time and at a temperature sufficient to react the support and the alkyl magnesium compound; (3) reacting, by heating, the product of (2) with a hydrocarbylhydrocarbyloxysilane compound; (4) reacting, by heating, the product of (3) with a titanium compound that contains a halide; or (5) reacting the product of (2) with the reaction product of a hydrocarbylhydrocarbyloxysilane compound and a titanium compound that contains a halide; and (6) activating the catalyst product of (4) or (5) with a cocatalyst comprising hydrogen or an alkyl lithium, alkyl magnesium, alkyl aluminum, alkyl aluminum halide or alkyl zinc.
Crotty et al. "Properties of Superior Strength Hexene Film Resins", Antec, 193, pp. 1210 describes the properties of superior strength hexene copolymer resins that are prepared by the Unipol process. These resins reportedly yield films with exceptional strength properties (impact and tear strength) that are significantly higher than the standard hexene products and even higher than achieved with commercially available octene copolymers. At the same time, the resins show little or no difference in processability from standard LLDPE.
The actual physical structures of polymers and abundant changes to same under various conditions is difficult to measure precisely and is commonly done indirectly. Rheology is often used in this regard, being especially suited to study the physical changes of polymers. Specifically, rheology deals with the deformation and flow of a polymer. Data so generated is used to provide information regarding the processability and even structural characterizations of the polymer.
One rheological method that is typically used is conventional, high shear modification wherein disentanglement of the polymer or copolymer chains occur. If a polymer or copolymer melt is sheared mechanically, the melt may be processed in a less elastic state or possibly less viscous state than the initial resin. Effects of shear modification are typically manifested by changes in die swell, die entrance pressure losses, normal stresses and flow defects such as sharkskin surfaces and melt fracture.
Although shear modification has been observed in LDPE, wherein disentanglement of the long chain branching of the polymer can readily occur, there was contention as to whether LLDPE could be shear modified. The question was answered in an article by The, et al. entitled "Shear Modification of Linear Low Density Polyethylene", Plastics and Rubber Processing and Applications, Vol. 4, No. 2, pg. 157 (1984). In this article, LLDPE was shear modified by preshearing the LLDPE resin under high shear conditions (&gt;3.9 sec.sup.-1) in an extruder. This study indicated that shear modification of the LLDPE polymer causes disentanglement to occur in the extruder, and that the relatively, disentangled polymer can be restored to a more highly elastic, entangled state by subjecting the melt to annealing or dissolving the shear modified polymer in a solvent.
Another rheological technique employed in the prior art to determine the physical characteristics of a polymer is to measure the polydispersity or melt elasticity, ER, of the polymer melt. This technique is described in an article by R. Shroff, et al. entitled "New Measures of Polydispersity from Rheological Data on Polymer Melts", J. Applied Polymer Science, Vol. 57, pp. 1605-1626 (1995).
Using this rheological technique (ER calculation), prior art ethylene copolymer resins, such as described in The, et al., exhibit conventional melt elastic behavior in both the unsheared pelletized and sheared pelletized states. In the unsheared state, the ER values of prior art ethylene copolymers remain substantially unchanged in going from the powder to pellet form. Moreover, no change in ER is observed in dissolving the pellet in an organic solvent.
As to the shear modified forms, prior art polymers exhibit a decrease in melt elasticity upon shear modification of the pelletized form. This signifies that the entanglement density of the polymer decreases. Upon dissolution of the shear modified form in an organic solvent, an increase in melt elasticity is observed with prior art ethylene copolymers. This increase in melt elasticity signifies a reversion of the polymer back to an entangled state.
In prior art ethylene copolymers, no polymeric networks, i.e. systems of interconnected macromolecular chains, are present. This is verified by the above melt elastic behavior of prior art ethylene copolymers. As is known to those skilled in the art, the presence of network structures in polymers often provides polymers having improved properties. It is emphasized that while network structures are common in styrene-butadiene-styrene (SBS) block copolymers--See F. Morrison, et al., "Flow-Induced Structure and Rheology of a Triblock Copolymer", J. Appl. Polymer Sci., Vol. 33, 1585-1600 (1987)--they are not known in LLDPE resins, until the advent of the present invention.
The attractiveness of obtaining a network structure in LLDPE resins, however, must be tempered by the desirability of utilizing an ethylene polymerization catalyst that provides high activity. Thus, a catalyst and catalyst system which produces a LLDPE exhibiting a network structure which, in addition, provides high catalytic activity is an aim long sought in this art.